EVIDENCE OF EVOLUTION Importance of Variation Among Species

GENETIC VARIATION Definition: differences among offspring due to the unique combination of traits inherited from parents Importance: 1.May give individuals an advantage over others for the struggle over resources 2.Increases the likelihood that some members of a species will survive if environmental changes occur

SOURCES OF VARIATION Variations often occur due to the following: Mutations Genetic Shuffling Geographic Isolation Reproductive Isolation

MUTATIONS Definition: change in the sequence of a DNA molecule Occur due to one of the following: 1.Random changes that cannot be predicted 2.Exposure of an organism to radiation, chemicals, or viruses

CHARACTERISTICS OF MUTATIONS Mutations are only passed on to offspring if they occur in sex cells Usually show up as recessive traits Can be favorable OR unfavorable, or have no effect on the organism at all

GENETIC SHUFFLING Recombination: chromosome pairs separate during meiosis and recombine during fertilization Crossing-over: portions of chromosomes exchanging places to produce new gene combinations

GEOGRAPHIC ISOLATION Definition: isolation of organisms by mountains, canyons, rivers, highways, climates, or other organisms Result: no interbreeding between the two groups, causing gene pools to diverge Kaibab Squirrel Abert Squirrel

REPRODUCTIVE ISOLATION Caused by geographic isolation Species have changed so much they can no longer interbreed, even if the geographic barriers are removed Result—formation of new species, or speciation

Different species of bowerbird construct elaborate bowers and decorate them with different colors in order to woo females. The Satin Bowerbird (left) builds a channel between upright sticks, and decorates with bright blue objects, while the MacGregor’s Bowerbird (right) builds a tall tower of sticks and decorates with bits of charcoal. Evolutionary changes in mating rituals, such as bower construction, can contribute to speciation.